Water, wastewater, and industrial filtration units typically have an underdrain system for supporting filter media and spacing the filter media apart from the bottom of the filter. In addition to providing support for the filter media, the underdrain system serves two primary purposes: to collect the filtered water that passes through the media and to uniformly distribute backwash water, backwash air, or a combination of both, across the filter.
Formations of support blocks are often used construct the underdrain system. The support blocks may be made of pre-cast concrete blocks or concrete filled plastic jacketed blocks. The high-strength concrete blocks are placed side-by-side and end-to-end in the bottom of the filter to form a “false bottom.” The blocks provide support for the filter media and are shaped to collect the filtered water that passes through the media.
In operation, a media filter will periodically require cleaning by backwashing the filter. In addition to shaping for collection, the blocks have conduits, commonly known as laterals, to allow for the passage of air or water used as a backwash. Piping, such as air headers, is part of the backwash air distribution system. Backwash water is brought into the system by flumes. Air headers bring in pressurized air, which is distributed into the laterals by pipes from the top of the headers, through the flumes and into the laterals.
During backwash operations, air is forced through the air distribution pipes and into the air laterals of the underdrain blocks. The pressurized air can be used as a backwash or combined with water for an air/water backwash. The pressurized air/water combination causes the filtered water to be passed upward through the media with sufficient velocity to prevent filter problems such as mud balls, filter cracking, agglomeration buildup on the media grains, and inactive areas within the filter. The underdrain blocks also form a barrier to physically separate the filter media from the air distribution pipes underneath the blocks. Separating the filter media from the air distribution pipes prevents the finer particles of the filter media from clogging of the orifices in the pipes.
Problems can occur during backwash because the upward pressure of the air and/or water against the blocks is too forceful. Sometimes the pressure can fall within a range of 2 psi to 6 psi. Such strong pressure tends to dislocate the blocks, a process known as uplift. When uplift occurs, filter media drops onto the pipes and damages the infrastructure. Repairing such damage is costly. In addition, the costs of emptying the filter and realigning the blocks are also expensive. The process is labor intensive and requires substantial downtime of the filter. As a consequence, there is a need for a support system that resists uplift.
In the existing systems for filter media support, several types of support blocks systems have been used. In U.S. Pat. No. 6,143,188 to Jantsch et al., grout is used to fill the gaps between the blocks. Other systems simply rely on the weight of the blocks to prevent block movement. In U.S. Pat. No. 6,325,931 to Roberts, side rails function to help align the blocks side-to-side in the filter basin. The side rails on one side are offset from the side rails on the other side to allow the rails to overlap with the rails of an adjacent block to prevent uplift.
One further example of an underdrain system, U.S. Pat. No. 5,160,614 to Brown, comprises a series of modular, interconnected air duct blocks. The row of air duct blocks extends transversely to a plurality of laterals. The laterals are defined by the connection of individual underdrain blocks in parallel adjacent rows. The air duct blocks are interconnected such that interiors of the separate air duct blocks are in fluid communication with one another to supply backwash gas through a single source of gas supply.
Existing underdrain block designs offer only a limited amount of backwash stability and some are difficult to assemble. Because stability in the support system is important to avoid damage to the infrastructure, there is a need for an improved underdrain system that is easily assembled and will limit the movement of the underdrain blocks during backwash procedures in both the vertical and horizontal directions.